The Nissan Leaf's climate control unit does not allow you to run the fan or switch to window defrost/defog mode without automatically turning on the battery draining heater.In humid or cool weather it is desirable to run the fan at a low setting to keep the windows clear.

Here are three different alternative methods to allow you to run in fan-only mode.

All of the variants below rely on the same underlying principle:They trick the climate control module into thinking that the heater fluid is already heated up to around 160F (70C). So it no longer turns on the PTC heater. No energy is wasted when running the fan or switching to defrost mode. The rest of the system will continue to operate normally.

Use this method if you hardly ever need the heater. It will remain always off.

Make sure the car is unplugged and turned off. Open the car's hood. On the drivers side you see the lead acid battery. Look over the top of the battery, down in the direction of the orange cables. Just above these cables are some insulated pipes with a T-junction in the middle. There may be a white stripe on top of it. The heater fluid sensor is is located on the bottom of that junction.

Press the tap on top of that plug and pull towards the bottom to unplug the sensor. Take a 100 Ohm resistor (82 to 180 Ohms is fine) and insert it into the two holes of the plug. You could bend each of the two resistor wires to V-shape before inserting it so the resistor can be fully inserted.

It may be a good idea to use some insulating tape to protect both the plug and the jack on the sensor from moisture and corrosion.

Caution: You should avoid leaving the connection open or shorted out while the car is turned on. This will cause NO damage but it will log an error (DTC codes B277C or B277D). It may be just temporarily reported as a warning but it is unclear if it will automatically go away or if the car insists on this being cleared by a dealer. (EVSE DTCs for example just cause a display of the yellow warning symbol. This will automatically go away when the car is turned off and on again.)

This method requires a 330 Ohm resistor (anything from 180 to 390 Ohms should be fine), a switch, two wire taps, and two pieces of wire.

Please make sure that the car is unplugged and turned off. Do not turn on the car while any of the connectors are unplugged.

Remove the cover of the climate control unit in the center console. There are no tools required. It may be a good idea to put a blanket over the cup holders and drive selector.

If you have the service manual this is on page IP-18 removal instructions for "cluster lid C".Or follow this installation video provided by Ingineer.

The 40 pin connector M50 is located on the back of the climate control unit "A/C Auto Amp". Unplug the connector. There's no need to unscrew the A/C unit.

On the bottom row of the connector locate the red ground wire on pin 30. A little bit further to the left the brown wire on pin 37 is the heater fluid sensor input. Put the wire taps on these two wires and connect them to the two new pieces of wire.

If you drop the wires down the opening on the left hand side of the center console they come out in the vicinity of the gas pedal. This would be an easy and inconspicuous place to put the switch.

At the end of these to wires connect the switch IN SERIES with the 330 Ohm resistor.

Turning the switch on will connect the resistor in parallel with the sensor therefore lowering the total resistance. This makes the climate control unit think that the heating fluid is a lot warmer than it really is.

Note that when the climate control unit was left unplugged for more that 20 mins or so, the defog light will blink for around 15 secs when the car is turned on for the first time. Don't panic, this is normal.

Now this is the deluxe method. If the A/C light is on, the climate control behaves normally. But if the A/C light is off, then the heater will never be turned on.Note that regardless of the manual A/C setting, whenever you preheat your car with the climate control timer, the A/C light always comes on. So preheating always works without manual intervention.

You need any N-Channel MOSFET (1N7000 or BS170, ...), any diode (1N4148, 1N4001, ... a Schottky diode would be even better), one 100nF capacitor, two 1 MOhm, a 330 Ohm, a 220K resister, and some 32AWG magnet wire (aka 0.2mm enameled copper wire).

Please make sure that the car is unplugged and turned off.

Do not turn on the car while any of the connectors are unplugged.

Remove the cover of the climate control unit in the center console. There are no tools required. It may be a good idea to put a blanket over the cup holders and drive selector.

If you have the service manual this is on page IP-18 removal instructions for "cluster lid C".Or follow this installation video provided by Ingineer.

Also remove the climate control unit.

Flip it over and remove the 6 PH screws:

Remove the back cover:

Grab the PCB by the white M50 connector and just take it out. There are no further screws or clips. This is also a safe procedure as the climate control buttons in the case are secured, they will not fall out.

In the upper left corner you will see the solder side of the M50 connector. LED5 is the A/C light. The LED6 is in parallel. It is not populated, so it's pads are free and closer to the connector. Very convenient.

The left pad of LED6 is connected to ground by a transistor whenever the LED is turned on. If the LED is off you will get 12 Volts thru the resistor and LED.

You can use double sided tape to secure the MOSFET, here a 1N7000 in a TO-92 package, and the capacitor. The rest is just soldered on the fly like that.

If you use another MOSFET it may have a different pin-out. The picture below indicates Source, Drain, and the Gate.Please note that the front cover with the button does have very close contact to the PCB around all the buttons and LEDS. Do not place any components below the MOSFET. There is lots of head room above the connector. Always make sure that the front cover still fits over the board!This is also the reason why magnet wire is required. This has to cross one of those contact points. So a regular insulated wire would be too thick. If you have never used magnet wire before: It is insulated with some paint that will be burned off by soldering.

Note that when the climate control unit was left unplugged for more that 20 mins or so, the defrost light will blink for around 15 secs when the car is turned on for the first time. Don't panic, this is normal.

Here are the schematics, the yellow box contains the newly added components:

Please note that the above circuit has evolved and was influenced by the components that were found in the desk drawer. There are many other ways to solve the problem.

The first approach just had the MOSFET and the 330 Ohm R1 resistor. The MOSFET gate was connected to the LED. When it is off the 12 Volts will switch on the MOSFET and put the R1 resistor in parallel with the heating fluid sensor to fake the high temperature reading. That worked perfectly to switch the heater on/off until the car lights were turned on. The climate control unit then dims the LEDs. It does that with 200 Hz PWM signal with a 10% active low duty cycle. This has kept the heater off even when A/C was on. So to fix this, the rest of the circuit was added to remove that signal. D1 discharges the capacitor quickly, while R2 charges it very slowly. The voltage divider R3/R4 lowers the voltage below the MOSFET ~1 Volt gate threshold. If the signal remains high for a longer period than the 5 ms of the PWM signal, then the MOSFET turns on. You might be able to do without the two resistors if you use a Schottky diode instead of a regular one, as this lowers the voltage by .5 Volts and/or use a MOSFET with a higher threshold.

SMD components may have been a better choice. You could also use a 4016 CMOS IC with four analog switches to solve the problem. The neat thing here is the 300 Ohm on-resistance, so R1 is not required. Battery voltage (13.8 Volts) is on pin 31 of the M50 connector. The complete pin-out is on page HAC-48 of the service manual.

If you prefer to solder the modification on the component side of the PCB you can use the right side of the unpopulated C159 pads which connect to the T20 LED driver transistor.

Here's a table that shows the resistance values of the heating fluid sensor (HAC-13 of the service manual), the temperature in degrees Celcius and the voltages on a 2012 Leaf. Note that the 2011 service manual page HAC-39 lists different voltages. It appears that the current resistor on the 5 Volts voltage divider for the sensor is 1000 Ohm. If the manual is correct it may have been 390 Ohms in the 2011 models. For the purposes of this modification it is largely irrelevant. Just to let you know in case you measure the circuit. For room temperature the voltage is 2.5 Volts and when the 330 Ohm resistor is active it is around 1 Volt.

BetterLeaf wrote:The Nissan Leaf's climate control unit does not allow you to run the fan or switch to window defrost/defog mode without automatically turning on the battery draining heater.In humid or cool weather it is desirable to run the fan at a low setting to keep the windows clear.

Here are three different alternative methods to allow you to run in fan-only mode.

Thank you for posting this. Looks like you went for the deluxe version. I will probably atempt that too.

I would also be interested in being able to run defog in recirculation, I realy miss having this option.

I pulled mine out this evening and did the "deluxe" version. It took me about one hour from start to finish and I already had the required components or close-enough equivalents in my parts box... I did decide to do it on the other (component) side of the board though since that looked cleaner. and had less clearance issues... Works like a champ! Thanks for the idea!

adric22 wrote:I think the deluxe version makes the most sense.. I may have to perform this at some point when I get some time like a long weekend.

Last edited by TomT on Thu Feb 07, 2013 10:13 am, edited 1 time in total.

Next Job: Figure out how to use AC when pre-conditioning. Here in the UK (with all that jolly-old rain) pre-heating invariably mists up the inside of the car. If only we could figure out a way to run the compressor to dehumidify it too...